Screw thread grinding machine lead compensator



Feb. 25, 1941. F. B. HUGHES SCREW THREAD GRINDING MACHINE LEAD COMPENSATOR Filed March 21, 1940 3 Sheets-Sheet 1 INVENTOR ATTOR Feb. 25, 1941, HUGHES 2,232,704

SCREW THREAD GRINDING MACHINE LEAD COMPENSATOR Filed March 21, 1940 s sheet-5 am 2 Q: 1 r) Q l \q z 1 N I I W l ,M T? 31 f 0 J; l

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scanw THREAD GRINDING mourns LEAD comrnusnoa Filed March 21, 1940 3 Sheets-Sheet s um/u Paul /lea Patented Feb. 25, 1941 This invention relates to a screw thread grinding machine lead compensator, and has for an object the production ofscrew threads of extremely high accuracy.

As is well known to those skilled in the art, a great deal of progress has been made in the rapid production of ground screw threads, all the dimensions of which can easily be held within very closelimits, except that of pitch or lead. In many types of work it is extremely important that the pitch or lead be held within the closest possible limits of error, or even better, be held at absolute accuracy. Absolute accuracy is extremely desirable if possible in such cases as in the control 1 screw of a fire control mechanism, for even the slightest inaccuracy of such a screw becomes greatly magnified by the time the projectile reaches its target.

In the grinding of a screw thread variations 20. and errors in lead or pitch may be caused by temperature changes, error in the lead screw, misalignment of the lead screw and the work, by the gearing and by a change in friction. Such sources of error may tend to nullify one another or they may supplement one another, so that the effect is a condition that must be corrected for the set up being used. This invention will correct such errors for any setup in the grinding machine, and in addition can be used to change the lead on a screw for a long or a short distance so that the mating part can be made to bind with any degree of tightness desired.

With the foregoing and other objects in view, the invention consists in the construction, combination and arrangement of parts hereinafter described and illustrated in the drawings, in which,

Fig. 1 is a front phantom elevation of a thread 4 grinder to which the attachment of this invention has been applied;

Fig. 2 is a horizontal longitudinal section on line 22 of Fig. 1;

Fig. 3 is a front elevation of the attachment of line 33 of Fig. 2;

Fig. 4 is a horizontal vertical cross-section on line 5-5 of Fig. 1; Fig. 5 is a horizontal sectional view on line 5-5 of Fig. 3; and,

Fig. 6 is a longitudinal sectional portion of a screw thread showing a trial grind short of standard pitch or lead pP.

There is shown at It a known form of thread grinder having a grinding wheel It for grinding the threads I2 of a thread screw 53,

PATENT OFFICE SCREW THREAD GRINDING MACHINE LEAD- COMPENSATOB Francis B. Hughes, Washington, D. C. Application March 21, 1940, Serial No. 325,190 4 Claims. (01. 51-95) (Granted under the act of March a, 1883, as

amended April 30, 1928; 370 0. G. 757) held between head stock 14 and tail stock l5 of the thread grinder III, the thread screw l3 being intended for use in a fire control mechanism or other mechanism requiring a thread screw of as absolute accuracy as possible. 5

The stocks I4 and I5 are 'adjustably mounted on the table IS in the usual manner, while the tail stock i5 is rotated by means of the'sh'aft I! connected through the gear box l8 to the lead screw 20, which extends through a nut 2! held to between collars, not shown, in the table bed 22.

A motor, not shown, supplies power through the gear box is and other gearing, not shown,-to rotate the grinding wheel H, the tail stock shaft H and the lead screw 20 so as to operate the grindit ing wheel and to control the rotation of the thread screw or work It and the advance of the table l6 over the bed 2'2 in proper synchronism. Obviously, the grinding wheel H may be rotated by a separate source of power, but all of the 20 mechanism thus far described is old and well known and is not per se this invention.

In setting up the work it is necessary to adjust the position of the work relative to the grinding wheel, and this is done by shifting the table i6 25 relative to the bed 22. The rotation of the lead screw 20 in one direction or another through the nut 2i advances or retracts the table. Rotation of the nut 2| while the lead screw 20 is stationary would likewise have the same effect of advancing 30 or retracting the table. The nutJZI is provided with a worm wheel screw 23 which meshes with a orm screw 26 keyed to a shaft 25 journalled to the bearing 26 in the bed 22, a square end 27 of the shaft 25 extending through the front of the 35 bed 22.

A crank handle, no. shown, may be attached to the square end 21 to rotate the nut 25 and thus advance or retract the relative position of the table it to the bed 22 and thus of the threads i2 40 of the work {3 to' the grinding wheel 0 l, these details of mechanism just described, likewise being old and conventional.

As is obvious in the various mechanical details just described for positioning and advanc- 45 ingthe workrelative to the grinder, inaccuracies may creep in in spite of the greatest care, or such inaccuracies may be due even to changes in friction or in temperature anywhere along the gear train connecting the lead screw 20 through the gear box I8 to the tail stock shaft ll. Likewise, as the work is operated on, the temperature increases, causing uneven expansion, which may cause additional errors to creep in. During operation the shaft 20 rotates through the normally 5 a bracket 51 mounted on the table I6.

stationary nut 2| to position the thread |2 of the work |3 relative to the grinding wheel The work |3 when put in position between the tail and head stocks l4 and I5 is already provided with a preformed rough thread. A trial cut is taken through the threads |2 of the work l3 and then the screw or tap I3 is removed and inspected for accuracy of pitch or lead. Any inaccuracy such as at p in Fig. 6 becomes magnified as at P, and this is easily detected in a convential comparator such as one which makes use of the well known J ohannasen blocks, and shows up inaccuracies as little as ten thousandths of an inch.

Obviously, any inaccuracy will appear in the screw thread |2 if due to temperature changes or for any other reason, the lead screw 28 advances the table l6 either too rapidly or too slowly, relative to the bed 22. Inasmuch as the shaft 25 by turning nut 2| advances or retracts the table, it is possible that any inaccuracy in the speed of rotation of the lead screw 28 may be compensated for by rotation of the nut 2| at the proper rate to exactly cancel out such an inaccuracy. Rotation of the nut 2| at the proper rate by means of the shaft 25 becomes possible as a result of-this invention. This is done by providing a gear wheel 28 which is journalled on a collar 38 carried by the shaft 21 and secured to the shaft 21 through set screws 3| and a flange collar 32, which is keyed at 33 to the shaft 21. Two of the screws 3| are provided with looking clamps 34, one end 35 of each clamp 34 extending into slots 36 in the face of collar 38 and the other end 31 extending over the face of the gear wheel 28. When the screws 3| are left loosein the flanged collar 32 the shaft 25 may be rotated free of the gear 28 as is necessary in the original setting up of the position of the table and the bed, as when the crank handle is placed on the square end 21 of the shaft 25.

A vertical rack rod 38 is provided with rack teeth 40 held in mesh with the gear 28 by means oi a bearing plate 4| secured by stud screws 42 through the plate 43 to the bed 22. As shown in Fig. 5, this rod bearing plate may be made of two parts, consisting of a rear plate 44 provided with an angle 45, through which the rack rod 38 extends and is held in position by the front plate 46. Obviously, with such screws 3| tightened to lock the gear 28 to shaft 25, vertical movement of the rack rod in either direction will rotate the nut 2| to advance or retard the motion of the table over the bed. To cause the vertical movement in exactly the desired amount, the upper end of the rack rod 38 is provided with a pin 41 carrying a roller 48 which fits within the slot 58 of a sine bar 5|. This sine bar 5| is pivoted at one end by a pin 52 secured to the table IS. The other end of the sine bar 5| is provided with rack teeth 53 in mesh with a worm gear 54 on a shaft 55 provided with a knurled knob 56 and journalled in The rack tooth end 53 of sine bar 5| is provided with a graduated face 58, zero position 68 being such that the slot 58 is exactly horizontal, while positive and negative graduations 6| extend above and below the zero position. An arcuate slot 62 is also provided in this rack end 53 and has a locking set screw 63 extending therethrough for locking the sine bar 5| at any desired angle after such angle has been selected by rotation of the rack worm gear 54.

In operation, each graduation 6| serves to compensate for a certain degree of inaccuracy measured in ten thousandths more or less, as may be desired. Any inaccuracies as pP in the work |3 are compensated by adjusting the angle of the sine bar 5| according to the graduations 6| by means of the knurled handle 56 on the worm shaft 55. As the table |6 moves relative to the bed 22 it carries the sine bar 5| with it, properly adjusted, as already described, and located in adjusted position by tightening the stud screw 63, the gear 28 having likewise been locked to the shaft 25 by tightening the locking clamps 34 by means of set screws 3|. If no inaccuracy is present the sine bar 5| is set at the zero position 88, in which case there will be no vertical movement of the rack rod 38, and hence no rotation of the nut 2|. If any inaccuracy is present in the pitch p? of the work, the angle of the sine bar 5| is accordingly adjusted plus or minus, as the case may be. Then as the table I6 is moved by rotation of the lead screw 28. the trackway provided by the slot 58 of the rack rod 38 will act on the roller 48 of pin 41 to raise or lower the rack bar 38, as the case may be, and thus correspondingly rotate the nut 2| on the lead'screw 28 to advance or retard the table, and thus compensate for any error.

Although the invention has been described with relation to a particular screw grinder, it is obvious that -it is equally applicable to any machine wherein a lead screw is employed for relative motion of a table and bed, and, furthermore, it is likewise obvious that the slot 58 instead of being straight, as in the present attachment, might have a curved portion, more or less, or be partially curved and partially straight, and thus act as a camway to introduce any desired changes in pitch. Thus, for instance, it may be desired to provide a work having threads at one end of a slightly different pitch than at the other end, whereby the screw would fit easily until almost home in operative position, and would then bind as much as desired. Such result on the work would easily be provided by proper shaping of the slot 58.

Other modifications and changes in the proportions and arrangement of the parts may be made by those skilled in the art without departing from the nature of the invention, within the scope of what is hereinafter claimed.

The invention described herein may be manufactured and/or used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

Having thus set forth and disclosed the nature of this invention, what is claimed is:

1. In a machine having a bed on which a working tool is located, a table relatively movable over the bed carrying work to be operated on by the working tool, a lead screw for moving the table relative to the bed while the tool is operating on the work, and a nut through which said lead screw is operated, said nut being independently rotatable to also move said table over said bed, means for rotating said nut to advance or retract the table to locate it relative to the bed, and means for controlling the rotation of said nut rotating means as said table is. moved over said bed by said lead screw, said last mentioned means being adjustable to compensate for any error in the relative motion of the table to the bed by the lead screw, said last mentioned means comprising a nut rotating shaft, a 'gear 2,232,704 on said shaft and means actuated by the move:

ment of said table to cause said gear to rotate said shaft.

2. In a machine having abed on which a working tool is located, a table relatively movable over the bed carrying work to be operated on by the working tool, a lead screw for moving the table relative to the bed while the tool is operating on the work, and a nut through which said lead screw is operated, said nut being independently rotatable to also move said table over said bed, means for rotating said nut to adment of said table to cause said gear to rotatesaid shaft, said table actuated means comprising a rack meshed with said gear, and an adjustable cam bar mounted on said table for operating said rack, said rack having a pin and slot connection to said cam bar.

3. In a machine having a bed on which a working tool is located, a table relatively movable over the bed carrying work to be operated on by the working tool, a lead screw for moving the table relative to the bed while the tool is operating on the work, a nut through which said lead screw is operated. said nut being independently rotatable to also move said table over said bed, means for rotating said nut to advance or retract the table to locate it relative to the bed,

and means for controlling the rotation of said nut rotating means as said table is moved over said bed by said lead screw, said last mentioned means being adjustable to compensate for any error in the relative motion of the table to the bed by the lead screw, said last mentioned means beingactuated by the movement of said table over said bed, said nut rotating means comprising a shaft, a worm and gear connection between said shaft and said nut, said last mentioned means comprising a gear on said nut rotating shaft, a rack bar meshed with said gear, a cam bar adjustably mounted on said table, and a pin and slot connection between said rack bar and said cam bar.

4. In a device for compensating errors of a thread forming machine, a feed screw member provided with a thread extendingtherealong, a nut member mounted on said screw member to provide relative longitudinal movement of the nut member to the screw member upon rotation of either member, means to support a work piece movable with one of said members, gearing connecting said work piece and one of said members and causing rotation ot the work piece and the member simultaneously, a rack bar, gear means to connect said rack bar to rotate the other of said members, a cam bar pin and'slot bers relative to the other of said members a desired amount to compensate for any error in their relative longitudinal movement.

FRANCIS B. HUGHES. 

